1. Field of the Invention
The present invention relates to a trigger circuit for a safety system which comprises an energy source for at least one trigger in a branch circuit, at least one response stepping switch which is driven into a first switch position or a second switch position depending on two delay values that are assigned to first and second conditions, and at least one selection switch which, dependent on the switch position of a status switch, connects the trigger to one of the switch contacts of the response stepping switch.
2. Description of the Prior Art
In a motor vehicle air bag system, for example, comprising a delay sensor that drives a response switch when a defined delay value is reached is a safety system of the type mentioned above. A motor vehicle is usually additionally equipped with safety belts. When an accident happens at low speed, the safety belt that is worn is normally adequate by itself to protect the occupants. Triggering the air bag is only needed given an accident at high speed. Such a safety system is disclosed in the German application No. 34 13 768, fully incorporated herein by this reference.
The response switch is thereby expanded to form a response step switch. Given an impact at a lower speed and, consequently, a smaller delay value, this response step switch is driven to a first switch contact position by a delay sensor (response case 1) and is driven to a second switch contact position given a greater delay value (response case 2).
The position of a status switch, a belt switch here, indicates whether the safety belt is attached. In accordance therewith, a selection switch is driven that connects the trigger for the air bag either to the first or to the second switch contact of the response step switch.
When the belt switch is closed, i.e. the belt is being worn, the selection switch is condition such that a triggering of the air bag only occurs in the response case 2.
When, by contrast, the belt switch is open, i.e. the belt is not being worn, the selection switch is conditioned such that a triggering occurs in the response case 1 and, therefore, the air bag assumes the protectie function of the belt that is not being worn.
In accordance with the art heretofore known, the control of the selection switch occurs by way of relays. For a proper operation of the trigger circuit, the switch position of the selection switch defined by the belt switch must be preserved in every case. A bistable relay suitable for this purpose, however, is not yet available due to the required impact resistance of 100 g and vibration resistance 5 g given a frequency of 20-30 Hz. These requirements are only met by relays that are held in their switch positions by continuously-present excitation energy. This, however, means a considerably, permanent energy consumption in the operating readiness of the trigger circuit.
A further problem is that a destruction or separation of the vehicle battery supplying the execution energy can occur in accidents before the triggering of the air bag. The switch position of the selection switch is therefore left to chance.